Bearing steel composition

ABSTRACT

A bearing steel consisting of 0.55 to 0.78 percent of carbon, 0.50 to 2.00 percent of chromium, 0.10 to 1.15 percent of manganese and 1.00 to 2.00 percent of silicon by weight with the balance iron, said steel having been spheroidized during annealing, and afterward, heated to an austenitizing temperature ranging from about 810* to 870* C. for a period of about 30 minutes to dissolve 0.35 to 0.55 percent by weight of carbon into austenite retaining 3 to 6 percent by weight of undissolved spheroidized iron carbide, liquid quenched and tempered at about 150* C.

United States Patent Monma et a1.

[ 51 May 16, 1972 [54] BEARING STEEL COMPOSITION [22] Filed: Oct. 14,1970 [21] Appl. No.: 80,646

Related U.S. Application Data [63] Continuation-impart of Ser. No.691,868, Dec. 19,

1967, abandoned.

[52] US. Cl. ..l48/36, 75/126, 75/126 Q, 148/144 [51] Int. Cl. ..C2ldl/00, C22c 39/14 [58] Field ofSearch ..75/126, 126 Q, 128; 148/31,148/36, 134,143,144

[ 56] References Cited UNITED STATES PATENTS 2,325,088 7/1943 Wright eta1. ..75/126 Q 2,753,260 7/1956 Payson ....75/126 Q 2,844,500 7/1958Peras.... ..75/l26Q 3,117,863 l/l964 Roberts et a1... ..75/126 3,155,55011/1964 Mitchell et al.... 148/134 3,298,827 1/1967 Jatczak ..75/1283,306,734 2/1967 Aksoy et a1. ..75/126 3,337,376 8/1967 Grange 148/1433,595,711 7/1971 Faunce et a1. .148/36 OTHER PUBLICATIONS Tool Steels,Roberts et al., ASM, 1962, pp. 321- 339.

Primary Examiner-Charles N. Lovell Attorney-Herman, Davidson & Berman[57] ABSTRACT A bearing steel consisting of 0.55 to 0.78 percent ofcarbon, 0.50 to 2.00 percent of chromium, 0.10 to 1.15 percent ofmanganese and 1.00 to 2.00 percent of silicon by weight with the balanceiron, said steel having been spheroidized during annealing, andafterward, heated to an austenitizing temperature ranging from about 810to 870 C. for a period of about 30 minutes to dissolve 0.35 to 0.55percent by weight of car bon into austenite retaining 3 to 6 percent byweight of undissolved spheroidized iron carbide, liquid quenched andtempered at about 150 C.

3 Claims, 13 Drawing Figures Pmmd my 16-, 1972 4 81:00 a-Sho 3 FIG.

FIG. 6.

0.45% CARBON X4000 O. 65 CARBON X4000 INVENTORS.

TOSH/RO YAMAM q BY I 6 M52 M W? ATTORNEYJ.

This application is a continuation-in-part of applicants previouslyfiled application Ser. No. 691,868, filed Dec. 19, 1967, now abandonedand entitled Bearing Steel Composition and Method of Manufacture.

This invention relates to steels usable for making bearings, and moreparticularly to an improved composition and method of heat treating suchsteels in which the carbon content is lower than normal and a particularproportion of the carbon is dissolved in austenite by introducing anaustenitizing step, the balance of undissolved carbon remaining asspheroidized iron carbide in martensite.

In the making of steel for bearings it is known that the carbon contentof the steel afiects the fatigue life, the compressive breakingstrength, and other important characteristics of the finished steelproduct. However, insofar as known to applicants, all conventional testsand research to discover the relation between the carbon content and theabove-named characteristics have failed to establish a distinctcorrelation, because the total carbon content appears in the finishedsteel in varying proportions both as dissolved carbon in austenite andundissolved iron carbide in martensite. Accordingly, a specific totalcarbon content results in varying characteristics depending on themethod of heat treatment and the variations in dissolved carbon andundissolved carbide.

It is a primary object of the present invention to provide a steelcomposition, or product, and method of manufacture in which the percentof carbon dissolved in austenite and the percent undissolved as carbideare predetermined to yield the optimum properties such as long fatiguelife and high compressive breaking strength.

It is another important object of the invention to overcome the researchproblem mentioned above and to determine what steel composition and heattreatment are most suitable to provide long fatigue life and highcompressive breaking strength in bearing steel.

Experimentally, according to the present invention, it has beendetermined that use of a less than normal quantity of carbon in therange from 0.55 to 0.78 percent by weight and the introduction of anaustenitizing step after spheroidizing annealing to dissolve 0.35 to0.55 percent of the carbon in austenite and leaving the balance ofundissolved carbon as spheroidized carbide in martensite yields abearing steel with optimum characteristics.

And furthermore, it has been found that longer fatigue life can beobtained by use of a greater percentage of silicon than usual, as forexample, a percentage ranging from 1.00 to 2.00 percent.

As a result of determining the best steel composition and process formanufacture, additional advantages have been gained, such as: thesoaking period is shortened because less large carbide appears in thefinished bearing steel; the conventional normalizing process may beomitted because less net carbide, proeutectoid cementite, exists becauseof the lower carbon content; the time period for spheroidizing annealingcan be shortened because of the reduction of net carbide; and the totalcost of steel production is lowered because of the shortening of thesteel fabrication processes.

Applicants have found that steel bearings attaining the foregoingobjects can be produced from steels consisting of carbon, silicon,manganese and chromium within restricted ranges. The desired propertiescan be obtained within the following ranges:

With the balance iron and residual impurities.

A preferred range within the foregoing is as follows:

Carbon O to 1.80%

0.6 Silicon 1.5

Manganese Chromium 0.50 to 2.00% With the balance iron and residualimpurities.

The most preferred range is as follows:

Carbon 0.65 to 0.70% Silicon 1.50 to 1.80% Manganese 0.10 to 1.15%Chromium 0.50 to 2.00%

With the balance iron and residual impurities.

The novel features that are considered characteristic of the inventionare set forth with particularity in the appended claims. The invention,itself, however, both as to composition and method, together withadditional objects and advantages thereof, will best be understood fromthe following description of specific embodiments when read inconnection with the accompanying drawings, wherein:

FIG. 1 is a diagram showing the relations between carbon content andhardness, and between carbon content and compressive breaking strengthin carbon steel samples, whose total carbon content is dissolved inmartensite matrix.

FIG. 2 is a diagram showing the relation between carbon content andfatigue life of the carbon steel samples.

FIG. 3 is a diagram showing the relations between undissolved carbideand fatigue life, and the compressive breaking strength and hardness insteel samples containing various quantities of carbon and about 1.50percent of chromium, about 0.45 percent of said carbon content beingdissolved in martensite matrix.

FIG. 4 is a diagram showing the relation between silicon content andfatigue life in steel samples containing about 0.45 percent of carbonall of which is dissolved in martensite matrix.

FIG. 5 is a diagram showing the relation betweenv silicon content andfatigue life in steel samples containing about 0.70 percent of carbonand about 1.45 percent of chromium, about 0.45 percent of said carboncontent being dissolved in martensite matrix.

FIGS. 6 to 13 are microphotographs of the steel samples used in plottingthe diagram illustrated in FIG. 3.

The following is pertinent as background information for the invention.Recent demand for hearing steel has greatly increased. The main alloyelements of such bearing steel are carbon, chromium, silicon, manganeseand iron. Depending on its use, the nature and size of bearing, thereare differences in content of carbon, chromium, silicon and manganese inhearing steel. The Japan Industrial Standard accepts three kinds ofbearing steel according to the following table:

TABLE I [Japan Industrial Standard for bearing steel] Percent Content ofDesignation carbon Chromium Silicon Manganese The industrial standardsfor other countries similarly designate bearing steels having a carboncontent of about 1 percent by weight as in the United States, GreatBritain, West Germany, France, and Sweden. Where lower carbon contentsteels are accepted for bearings they are usually for special purposesand are case hardened. Generally, a steel containing about 1 percent ofcarbon, 1.5 percent of chromium, 0.25 percent of silicon and 0.35percent of manganese, as AISI No. 52100 for example, is most commonlyused for bearing steel.

In conventional processes for manufacturing a bearing the steel is madeby (l) forming a melt, (2) degassing, (3) casting an ingot, (4) soaking,(5) rolling, (6) normalizing, and (7) spheroidizing. The bearing is thenfabricated by (8) turning, (9) quenching, (10) tempering, and l l)grinding or polishmg.

It is a well-known fact that the microstructure of bearing steel afterquenching is observed to contain Fe C (cementite) as when a steelcontaining 1 percent of carbon is spheroidized, all of the carbon existsas ferrous compound of iron, Fe C (cementite) in the amount of 15percent of the steel. Upon austenitizing of said steel as the quenchingtemperature, i.e., austenitizing temperature, is increased morecementite (Fe C) dissolves in austenite. According to austenitizingtemperature and austenitizing time, the quantity of this Fe C(undissolved carbide) varies from 15 percent to zero (all carbondissolved in matrix, and no Fe,,C remaining), and consequently thequantity of carbon dissolved in martensite matrix after quenching variesfrom zero to 1 percent. The longer the austenitizing time, the morecementite dissolves in the matrix, showing the same behavior as if theaustenitizing temperature were raised. That is, both the quantity ofcarbon dissolved in martensite matrix and undissolved carbide (Fe c)vary correlatively with austenitizing conditions (temperature and time).

l-leretofore in attempts to improve fatigue life and high compressivebreaking strength of bearing steel, studies were made only in relationto the austenitizing temperature, or time, and using only theconventional bearing steel whose main composition is almost limited toabout 1 percent of carbon and about 1.5 percent of chromium. In thesestudies, for reasons indicated above, it was impossible to find how muchcarbon should be dissolved in the matrix and how much undissolvedcarbide would be required for obtaining the most desirablecharacteristics of bearing steel.

The applicants have examined the relation between the character ofbearing steel and carbon content in martensite matrix, and between theformer and undissolved carbide. That is, separate investigations weredirected to the most desirable quantity of carbon dissolved inmartensite matrix and then to the most desirable quantity of undissolvedcarbide, for improving the fatigue life and compressive breakingstrength of the bearing.

To determine the most desirable quantity of carbon dissolved inmartensite matrix, samples of several kinds of carbon steels containingabout 0.20 to 0.80 percent of carbon were prepared according to TablesII and III.

Each sample was normalized and, omitting spheroidizing,

heated at the temperature higher than A transformation 5 The samples ofTable ll were then tested for compressive breaking strength (size oftest piece: outside diameter 20 mm, inside diameter mm, height 10 mm)and Rockwell hardness C, and the samples of Table III were tested forfatigue life.

As a result of the test of the samples of Table II, the effect of 10carbon content in martensite on compressive breaking tts sfl ess re erly ass ssin 9 by t data of Table II, wherein compressive breakingstrength and ;hardness is plotted as a function of carbon content. Asshown in FIG. 1, the hardness of carbon steel increases as the carbon 1content increases, and maximum compressive breaking strength occurs atabout 0.40 percent of carbon content diS-i solved in the matrix.

The fatigue life of the samples of Table III, obtained from a life testusing a Bearing Steel Life Tester, was measured in millions ofrevolutions of a ball on a test piece surface having a roughness ofabout 0.3 microns required to cause flaking by abnormal vibrationexceeding 10 microns.

From the data of Table III, the fatigue life is plotted as a function ofcarbon content in FIG. 2.

2 As shown in FIG. 2, both B life and B life had similar tendency, steelsamples containing 0.35 to 0.55 percent of carbon dissolved inmartensite matrix show long fatigue life, and especially, fatigue lifebecomes maximum at 0.40 to 0.50 percent carbon dissolved in martensitematric.

On the other hand, with a carbon content below 0.35 percent, thestrength of martensite is low, and also martensite is brittle when thecarbon content exceeds 0.55 percent.

It is concluded, therefore, that bearing steels containing 0.35 to 0.55percent of carbon dissolved in martensite matrix are desireable toprovide optimum fatigue life and compressive breaking strength, and thatbearing steels containing 0.40

to 0.50 percent of carbon dissolved in martensite matrix are the mostdesirable to provide above optimum properties.

Next, the influence of undissolved carbide (which exists in steel afterquenching) to the character of the bearing steel was examined. Eightkinds of steels whose carbon content varied from 0.45 to 1.17 percentwere prepared according to Table .1 a... .1 l-

TABLE II Austenitizing Tempering Compressive Chemical compositionbreaking Temp. Time Temp. Time strength Hardness Sample C S1 Mn C1 C.)Quin.) C.) (min) (ton) (I'IrC l 0. 20 t). 20 0. 22 0. 14 .100 30 150 )04. 10 29. 8 .2 0. 38 0. 23 0. 26 0. 12 800 30 150 JO 4. 66 54. 0 d 0. 500 2d 0. 30 0. 13 850 30 150 80 3. 80 58. 8 4 0.61 0 2 0. 20 0. 12 830 30150 '70 2. 57 (i1. 6 "1 0. H4 0 25 0. 24 0. 1). 800 30 151) I0 2. 40 04.5

TABLE I11 Fatigue life Austenitizing Tempering' (X10 Chemicalcomposition Temp. Time Temp. Time B10 B 0 Sample (3 S1 Mn C1 C.) (min.)0.) (min) life life 6 0. 23 0. 25 0. 3i) 0. 14 900 30 150 90 0. 042 0.27 7 O. 40 0. 26 0. 43 0. 14 860 30 150 90 0. 39 1. 70 8 0. 44 0. 20 0.34 0. 14 860 30 150 00 0. 5. 60 t) 0. 0. 21 O. 35 0. 14 850 30 150 90 0.22 0. 82 10 0. 0. 26 0. 44 0. 14 830 30 150 90 0. 060 0. 33 ll 0. 80 0.26 0. 44 0. 14 S00 30 150 J0 0. 041 0. 25

TABLE IV Chemical composition, percent A B C, D, E perper- Fig- SarnpleC Si Mn Cr C. Min C Min cent cent are 12 0. 45 0. 33 0. 39 1. 44 870 30150 90 0 0. 45 6 13 0. 55 0. 34 0. 41 1. 47 870 30 150 90 1. 5 0. 45 714 0. 0. 36 0. 38 1. 45 870 30 150 00 3 0. 45 8 15 0. 0. 35 0. 38 1. 50870 30 150 3. 5 0. 45 9 16 0. 78 0. 35 0. 38 1. 50 850 30 90 4. 8 0. 4510 17 0. 88 0. 35 0. 41 1. 48 840 30 150 90 6. 5 0. 45 11 I8 0. 5 0. 330. 3i) 1. 45 830 30 150 90 7. 8 0. 45 12 l!l 1.17 0. 35 0. 42 1. 45 81030 150 00 10. 6 0. 45 13 FPO??? These samples were normalized,spheroidizing annealed (mean carbide particle size: about 0.6 microns)and austenitized to contain about 0.45 percent of carbon in each matrix.Austenitizing temperature and time were changed for each sample, (seeColumn A). Since austenitizing tempera- 1 ture and time are changeablecorrelatively, the figures in Column A are only examples, and notlimiting. After austenizing, the samples were oil-quenched and tempered.Consequently, these samples contained the same amount (about 0.45percent) of carbon in the matrix and different amounts of undissolvedcarbide, (cementite). The amounts of the dissolved carbon in the matrixwere calculated by linear analysis counting the amount of undissolvedcarbide in comparison with the amount of carbon content in steel.

Samples in Table IV were tested for compressive breaking strength (sizeof test piece: outside diameter 25mm, inside diameter 10 mm, height 10mm), hardness and fatigue life.

The test results are set forth in Table V and depicted graphicallyin-FIG. 3.

TABLE V Compressive Fatigue life (X10 breaking strength Rockwell C 10life 50 life Sample (ton) hardness As apparent from FIG-3, thecharacteristics of the steel samples having carbon content in excess of0.65 percent are as follows:

The fatigue life is the longest within the range of 0.65 to 0.78 percentof carbon, but lowers remarkably as the carbon in the steel goes over0.78 percent, while the compressive breaking strength decreasesgradually with increase of the amount of carbon and hardness reverselyis higher, but not remarkably so.

The smaller the quantity of undissolved carbide, the finer the particlesof carbide. When carbon content exceeds 1 percent and the amount ofundissolved carbide exceeds 10 percent (see FIG. 13), particles of theundissolved carbide grow larger and their volume in matrix becomesgreat. Therefore, it is seen that fatigue life and compressive breakingstrength of the steel with large volume of undissolved carbide are low,because the undissolved carbide itself will cause origin of fracture, orinternal defect is induced by martensite of high carbon content producedon the surroundings of carbide.

Further, it is shown from FIG. 3, that the fatigue life and compressivebreaking strength of the steel samples containing less than 0.55 percentcarbon lower with the decrease of the amount of carbon in the steel, thequantity of undissolved carbide reduces and also hardness is lowerremarkably.

It is desirable, therefore, for bearing steel to contain more carbonthan the amount to be dissolved in the matrix. The

and Figure 2.

To have the carbon content just equal to that required to be dissolvedin the matrix results in dissolving the total carbon content during theaustenitizing treatment. If, however, any spheroidized carbon remains inthe matrix, the carbon dissolved in the matrix will be less than theamount required. Absence of spheroidized carbide in heat treatment isnot appropriate, as described above. therefore, it is preferably to havethe carbon content in excess of about 0.55 percent in order to dissolve0.35 percent of carbon in the matrix, retaining 3 to 6 percent ofundissolved carbide, to give easy control of dissolution of carbonduring austenitizing, and to stabilize the heat treatment.

That is, spheroidized carbide in the steel is cementite (Fe C) andcarbon percentage in cementite is 6.67 percent, the amount of carbon inthe carbide is calculated to be 3(percent) 6.67/100 0.2 percent, when 3percent of undissolved carbide in the matrix of martensite is retained.Consequently, the total carbon content in the steel is calculated to be0.55 percent, by adding 0.2 percent to 0.35 percent.

As is clear from the above, it is concluded that such bearing steel ascontains 0.35 to 0.55 percent of carbon dissolved in matrix and alsocontains between 3 to 6 percent of undissolved carbide after tempering,is most desirable.

Therefore, on considering both carbon content in martensite and theamount of undissolved carbide, the desired carbon range in bearing steelis limited with the range 0.55 to 0.78 percent, and the better carbonrange lies within 0.65 to 0.70 percent.

To add silicon greatly increases resistance to tempering of bearingsteel, and it has an advantage of preventing defect of cracking in thecourse of grinding after quenching and tempenng.

Applicants experimentally have found that fatigue life of bearing steelis improved by adding silicon. In examining the effect of siliconcontent on fatigue life, steel samples were tested containing noundissolved carbide in martensite matrix and also steel samples withundissolved carbide in martensite matrix.

Firstly, the samples containing 0.45 percent of carbon, all of which isdissolved in martensite matrix, were used, because such steels have thelongest fatigue life as mentioned above.

Each sample was austenitized at the temperature of 850 C. for 30 minutesto contain no undissolved carbide, then, quenched in oil and tempered at150 C. for 90 minutes. Each of the samples was then tested for fatiguelife.

The effect of silicon on fatigue life is clearly indicated by the dataof Table VI, and the fatigue life is plotted as a function of siliconcontent in FIG. 4.

TABLE VI Chemical composition (percent) Fatigue life (X10 C Si Mn Cr Blife B 0 life Sample 20 in Table VI and Figure 4 is equal to Sample 8 inTable III It is obvious from Table VI and FIG. 4 that the fatigue lifeof the tested samples is not improved when the silicon content varieswithin the range 0.25 to 0.80 percent and fatigue life inideal quantityof undissolved carbide was found to be 3 to 6 creases gr ly Over th rangof silicon content from 0.80

percent.

Moreover, as shown in FIG. 3, it is not desirable to have a carboncontent in excess of 0.95 percent because such carbon content remarkablydeteriorates fatigue life, while, as the carto 1.47 percent. Fatiguelife of Sample 23 having 1.47 percent of silicon is two times that ofsample 20 containing 0.20 percent of silicon. Since, as has been shown,the fatigue life of steel containing silicon in excess of 1 percentincreases rebon content in the steel is lowered, the steel has betterfatigue markably, therefore, a preferred range of silicon content islife and compressive breaking strength, and the steel with 0.78 percentof carbon exhibits the most advantageous improvement of these qualities.Further, the two steels with 0.65 percent and 0.70 percent of carbonhave the longest fatigue life and the highest compressive breakingstrength.

from about 1.00 to 2.00 percent.

Next, the fatigue life was examined by using steel samples with about0.70 percent of carbon and 1.45 percent of chromium, in which thesilicon content was varied from 0.35 to 1.99 percent.

Each of the samples was spheroidizing annealed (mean carbide particlesize: about 0.6 microns), austenitized at the temperature of 870 F. for30 minutes, quenched in oil and then tempered at 150 C. for 90 minutes.Consequently, each sample had about 0.45 percent of carbon contentdissolved in martensite and retained about 3.8 percent of undissolvedcarbide.

The results of the test analysis are shown in Table VII and illustratedgraphically in FIG. 5.

TABLE VII Chemical composition Fatigue life (pcrmn'it) (X Si Mn Cr Blife B 0 life 0 35 0. 38 1. 5O 2. 00 ll. 00 0 88 0. 41 1. 44 2. 31 12.33 "7 1 00 0. 38 1. 44 2. 75 14. 49 l. 2'.) O. 31) 1. 46 4. 40 22. 87l). 6!) 1. 50 0. 3 1. 45 5. 51] 27. 45 (l. 70 1.67 0.40 1. 45 5. 82 21].l 0. 70 l 80 0. 37 1. 43 5. 57 25. 58 32 0. 71 1 l1!) 0. 3!) 1. 46 4. 0120.08

No'ric. Sample 25 shown in 'lahleVll and Figure 5150111111110 Sample in'lnlilv 1V and l igul'v 3.

It is clearly depicted in FIG. 5 that the fatigue life of samples isvirtually changeless within silicon content range of 0.35 to 0.88percent, but increases gradually with increase of silicon proportion inthe range of 0.88 to 1.50 percent of silicon, and is the longest withinthe range of 1.50 to 1.80 percent of silicon. However, fatigue life ofSample 32 with silicon content 1.80 percent decreases somewhat, but isstill superior to that of Sample 27 containing 1.00 percent of silicon.Silicon in excess of 2.00 percent will lower the toughness of thebearing steel.

It is apparent from FIG. 4 and FIG. 5 that the fatigue life of steelswith 3.8 percent of undissolved carbide is longer than that of steelscontaining no undissolved carbide over the range of 0.20 to 1.99 percentof silicon content, and the steel containing about 1.00 to 2.00' percentof silicon, with undis'solved carbide, shows preferable fatigue life,and especially the desired range of silicon is about 1.50 to 1.80percent.

Chromium and manganese influence hardenability, resistance to tempering,wear resistance, ease of spheriodizing and the solution rate of carbidein austenite. In the martensite tate of about 150 C. of usual temperingtemperature, however, they scarcely affect fatigue life and compressivebreaking strength. They can be added adequately within the range ofcommon use (0.50 to 2.00 percent of chromium, 0.10 to 1.15 percent ofmanganese) according to use, nature and size of the bearing.

Vanadium, molybdenum and nickel are contained in aome bearing steels,however, such elements scarcely affect fatigue life and compressivebreaking strength of the steel in the state of martensite tempered 150C. of usual tempering temperature. Therefore, the steel of the presentinvention does not contain vanadium, molybdenum nor nickel.

From the foregoing, it is clearly apparent that a bearing steelcontaining 0.55 to 0.78 percent of carbon, 1.00 to 2.00 percent ofsilicon, 0.50 to 2.00 percent of chromium and 0.10 to 1.15 percent ofmanganese, wherein the steel has 0.35 to 0.55 percent of carbondissolved in martensite and retains 3 to 6 percent undissolved carbide,has the longest fatigue life, the highest compressive breaking strengthand suitable hardness at which the present invention aims.

EXAMPLE An example of a process for manufacturing steel and a ballbearing according to this invention, is as outlined in the followingsteps:

l. The steel whose chemical composition by weight is shown in Table VIIIbelow was melted in a l0-ton furnace:

TABLE VIII Percent Sample No 33 34 35 36 37 Chemical composition of thesteel:

Carbon 0. 95 0. 78 0. 78 0. 65 0. 55 Chromium 1. 44 1. 46 1. 32 l. 00 1.60 Manganese- 0. 41 0. 37 0. 25 0. 60 0. Silicon 0.25 0. 22 1. 52 1.63 1. 70 Impurities:

Phosphorus 0.013 0. 011 0.011 0. 010 0. 011 Sulfur 0. 007 0. 009 0. 001)0. 010 0. 009 Nickel. 0. 04 0.05 0.05 0. 06 0. 05 Copper 0. 07 0. 10 0.l1 0. 08 0. 09 The remainder: Iron 97. 82 17. 00 05. 95. 96 95. 09

2. Said melted steel was held in a vacuum atmosphere and degassed ofoxygen, hydrogen and nitrogen to improve the fatigue life of thebearing.

3. The melt was cast in an ingot mold of 2.5 tons and allowed tosolidify.

4. The ingot was soaked to decrease segregation of impurities under heattreatment at 1,265 c. for 5 hours, (conventional treatment is 15 hours).

5. The steel was rolled to a round bar of 65 mm.diameter to produce abearing numbered 6206 as final product.

6. Normalizing which is essential to conventional steels, was omitted,and, spheroidizing in annealing was applied soon after rolling.

7. The outer and inner rings of the bearings were turned from theannealed round bar on a lathe.

8. The cut rings were quenched and tempered under the condition oftemperature and time according to this invention, i.e., temperature: 810to 870 C., time: 30 minutes, dissolving about 0.35 to 0.55 percentcarbon in the matrix.

9. The rings were ground to eliminate the decarburized layer and topolish the track surface, and assembled to form the final product, aradial ball bearing No. 6206 having an outer diameter of 62 mm.,an innerdiameter of 30 mm. and a thickness of 16 mm.

The result of fatigue life test of the bearing steels on samples usingradialball bearing tester is shown in Table 1X.

Analysis of the data of Tables VIII and 1X is as follows: Fatigue lifeof Sample 34 (0.78 percent of carbon) is longer than that of Sample 33(0.95 percent of carbon), conventional bearing steel (ex. .118 SUJ2),and fatigue life of Sample 35 (0.78 percent of carbon, 1.52 percent ofsilicon) and Sample 36 (0.65 percent of carbon, 1.63 percent of silicon)are the longest. And also, fatigue life of Sample 37 (0.55 percent ofcarbon, 1.70 percent of silicon) is shorter than that of samples 35 or36, but is longer than that of Sample 33.

Applicants believe themselves to be the first to have discovered thatless than 0.35 percent of carbon dissolved in the matrix reducesstrength of the bearing steel, and that an excess of dissolved carbonabove 0.55 percent also is detrimental is causing brittleness in the'martensite. The further discovery that both fatigue life and compressivestrength are improved as the quantity of undissolved carbide decreasesappears to be novel. The introduction of a heat treatment in which thebearing steel is spheroidized in annealing and austenitized at thetemperature range of about 810 to 870 C. (at a quenching temperature),and consequently 0.35 to 0.55 percent of carbon is dissolved in themartensite matrix leaving a predetermined percent of undissolvedcarbide, is believed to constitute a new method of manufacture and a newcomposition of a bearing steel.

According to this invention, as described above, the soaking process forsteel can be shortened, the normalizing process can be omitted, and thespheroidizing annealing process can be shortened. Further, bearing steelis produced with fairly fine spheroidized cementite structure. Theconventional bearing steel specified by the Japan Industrial standard ishypoeutectoid steel having 0.95 to 1.10 percent of carbon content. If itis slowly cooled from rolling or forging temperature downward, there isfirstly precipitated proeutectoid cementite at austenite grain boundaryat temperature below Acm and then pearlite is formed at the point of Aand the structure becomes cementite and pearlite structure.Particularly, in a large sized bearing, the steel is slowly cooled forpreventing flakes and the net of its proeutectoid cementite developsmuch larger. Such thick proeutectoid cementite cannot be diminished byspheroidizing annealing in a later process, and, therefore, the steel isgenerally subjected to normalizing. In spite of such normalizing, it isquite difficult to prevent proeutectoid cementite newly precipitated byair cooling in such a large sized rolled and forged product. Also, inwater cooling, the operation always accompanies danger of quenchingcrack and requires much higher and more careful technique. F ormation ofsuch proeutectoid cementite is caused by high carbon content. Even inthe precipitation of a less thick proeutectoid cementite, itsdissolution rate in austenite is much slower than that of cementite inpearlite when it is heated over A point during the spheroidizingannealing process. If the steel is held a much longer time attemperature over A point during spheroidizing annealing in order todistribute spheroidized carbide finely and uniformly, the nucleus of thespheroidized cementite will decrease and the obtained particles ofspheroidized cementite will become coarse and cause origin of fracture.

Huge carbide content of bearing steel seems to be formed by eutecticreaction upon solidification of steel ingot when impurities such ascarbon and phosphorus increase in the melt among primary crystals duringcrystallization of proeutectic austenite, or is formed by easyprecipitation of carbide particularly in phosphor rich part because oflow solubility of carbide, when carbide beyond saturated solubilitylimit is produced by A transformation after solidification of steelingot. In either case huge carbide is certainly formed in thesegregation part of carbon, chromium and phosphor. In order to preventdefects occurring from the huge carbide, it is essential to consider theamount of carbon content which is the main cause of the formation ofhuge carbide as well as to dissolve the huge carbide or to diffuse thesegregation of elements of phosphor.

The present invention lowers the carbon content in steel to about 0.55to 0.78 percent, which does not develop proeutectoid cementite, and thiscementite can be diminished in spheroidizing annealing process withoutnormalizing processes. The solution of carbide into austenite ispromoted and controlled by simple adjustment of austenitizingtemperature and holding time. Fairly fine spheroidized carbide isobtained by spheroidizing annealing for a short period, and theformation of huge carbide is limited and, further, soaking time can beshortened.

Although certain specific embodiments of the invention ha been shown anddescribed, it is obvious that many modifications thereof are possible.The invention, therefore, is not to be restricted except insofar as 15necessitated by the prior art and by the spirit of the appended claims.

We claim:

1. A heat treated bearing steel consisting of 0.55 to 0.78 percent ofcarbon, 0.50 to 2.00 percent of chromium, 0.10 to 1.15 percent ofmanganese and 1.00 to 2.00 percent of silicon by weight with the balanceiron and residual impurities, said steel having 0.35 to 0.55 percent byweight of carbon dissolved in matrix of martensite and retaining 3 to 6percent by weight of undissolved spheroidized iron carbide said steelhaving been spheroidized during annealing, and afterward,heated to anaustenitizing temperature ranging from 810 to 870 C. for a period ofabout 30 minutes, liquid quenched and tempered at about C.

2. A bearing steel according to claim 1, wherein the carbon is 0.65 to0.78 percent, and silicon is 1.50 to 1.80 percent by weight.

3. A bearing steel according to claim 2, wherein the carbon is 0.65 to0.70 percent by weight.

2. A bearing steel according to claim 1, wherein the carbon is 0.65 to0.78 percent, and silicon is 1.50 to 1.80 percent by weight.
 3. Abearing steel according to claim 2, wherein the carbon is 0.65 to 0.70percent by weight.